Conventionally, breakers have heretofore been used as protecting devices for a secondary battery, a motor, and the like of various kinds of electrical machines. When a temperature of the secondary battery increases in excess during charging and discharging, or when abnormality occurs such as a case where excessive current flows through motors mounted in machines such as automotive vehicles and home electric appliances, the breaker shuts down electric current for protecting the secondary battery, the motors, and the like. To reliably ensure safety for machines, the breaker, which is used as such a protecting device, is required to precisely operate conjunction with a change in temperature.
Further, the breaker is likely to be used as the protecting device for the secondary battery and the like equipped in electrical machines such as a notebook personal computer, a tablet portable terminal device, and a compact multi-functional mobile phone called a smart phone. In such likelihood, in addition to enhanced safety mentioned above, the breaker is required to be miniaturized. In particular, there is a strong inclination for down-sized (thinner) portable information terminals on the part of users, and machines newly released by manufacturers have a remarkable tendency to be designed in miniaturization for enhanced superiority in design. Under such a background, there is a strong demand for further miniaturization of the breaker, which is mounted with the secondary battery as one component part of the portable information terminal device.
The breaker includes a thermally responsive element operating depending on a change in temperature for conducting or shutting off the flow of current. Patent document 1 discloses a breaker to which bimetal is applied as a thermally responsive element. Bimetal is an element composed of a stack of plate-shaped metallic materials of two kinds with different thermal expansion rates, which alters in shape depending on a change in temperature for controlling a conducting state of contacts. The breaker disclosed in the same document has component parts, such as a fixed piece (base terminal), a movable piece (movable arm), a thermally responsive element, and a PTC thermistor. These parts are accommodated in a resin case with terminals of the fixed piece and the movable piece being connected to an electric circuit of electrical machines in use.                Patent document 1: WO2011/105175.        